20-Hydroxyecdysone, a steroid hormone, induces evagination (morphogenesis) and differentiation of imaginal discs of Drosophila melanogaster in vitro under defined culture conditions. The cellular basis of evagination involves limited cell movement which appears to involve contractile proteins and to require an appropriate cytostructural state. 20-Hydroxyecdysone acts through the genome to produce evagination and induces increases in DNA, RNA and protein synthesis. Mutants have been recovered with discs incapable of evagination. After exposure of discs to 20-ecdysone for a period of time sufficient to cause evagination, removal of the hormone from the culture medium results in the formation of the chitin/protein-containing endocuticle. Using a radiolabelled ecdysone analog with high biological activity we propose to characterize ecdysone receptors and determine sites of specific intracellular binding of the hormone. We shall also investigate the structural basis of evagination with particular emphasis on the role of cell surface proteins. We will also characterize endocuticle structural proteins and study the synthesis of these proteins in discs. These studies will utilize mutants already obtained and involve the recovery of new mutants.